The invention relates generally to the field of image processing, and in particular to image enhancement and more particularly to image sharpening.
When images are subjected to intermediate processing steps such as printing and rescanning, the quality of the image may be reduced i.e., the sharpness of the images may be degraded, due to the limited modulation transfer function (MTF) of the intermediate processing steps. It is known to apply sharpening filters to images in an attempt to improve the appearance of the image either because they have been degraded by intermediate processing steps, or because they are being optimized for display on a particular medium. For example, digital images that are transmitted over the Internet are optimized for display on CRTs by applying a predetermined amount of sharpening. If the sharpened image is subsequently printed and rescanned, it is possible that more sharpening will be applied to the image in an attempt to improve its appearance. Unfortunately, if an image is oversharpened, its quality can be further reduced. Because of this, a digital image having an unknown processing history cannot be automatically sharpened by a predetermined amount prior to display. An operator is generally required to view the effects of different amounts of sharpening and select the one that produces the most improvement without reducing the quality of the image.
There is a need therefore for an improved method of sharpening digital images prior to display, and therefore to be able to know how much an image has been degraded due to the MTF of intermediate processing steps, and/or sharpened so that an optimal level of sharpening can be applied to the image when it is displayed.
The problem is solved according to the present invention by providing a digital image processing method including the steps of embedding a delta function in a digital image; subjecting the digital image to an intermediate processing step, the intermediate processing step having a modulation transfer function (MTF) that blurs the image; extracting the delta function from the processed digital image; and estimating the MTF from the recovered delta function. The blur is removed from the image by generating a sharpening filter that is substantially the inverse of the MTF and applying the sharpening filter to the blurred image to produce an improved image.
The processing method has the advantages of avoiding oversharpening an image and eliminating the need for an operator to select the amount of sharpening applied to an image for display.